My RV-8 is like the Concorde and SR-71!

[RV8Squaz]

OK, before you come to the conclusion that I've fallen off my rocker or been taking a bit much from the bottle, hear this...

While my airspeeds will never match, come close, or even remotely be in the same league as the aformentioned aircraft, my 200 hp, constant speed prop bird does exhibit a similar characteristic. That characteristic my RV-8 and all RVs have in common with the Concorde and SR-71 is... aerodynamic heating! Due to high airspeed, the friction of the air on the aircraft skin will cause a temperature rise. I know what your thinking... "that Jerry must be delusional."

So how did I come to this silly conclusion? Well I noticed my OAT never seemed accurate. I know folks have reported higher temps than actual and I know in some cases it was caused due to poor placement of the probe. However my probe is on the underside of my right wing in the NACA scoop for the copilot vent. It is well away from any potential exhaust. So why did my OAT consistently read about 4 deg C higher than what I though it should read even though I had calibrated it?

On one of those nice flying days (really, aren't they all nice), I noticed my OAT rose about 3 deg C while accelerating from 70 KIAS to about 170 KIAS despite maintaining level flight. Hmmm. I slowed down, the temp went down. Naahh, that couldn't be. It must be some weird air mass thing. Everytime I changed airspeed regardless of altitude, the temp followed. Now, that got my attention and got the electrons in my brain stirring.

So I turned to my favorite information resource... the internet! There I found calculators and formulas that reinforced what I was seeing. I figured I should have been seeing a 4-5 deg C rise? Why the difference? 4-5 deg C rise is from 0 to 170 KIAS, I was going from 70 to 170 KIAS. Thus at 70 KIAS I was already seeing a 1 deg C temp rise due to aerodynamic heating!

OK, sounds great. Can this really be true? Well today I was on final approach into ATL in one of my company's Boeing 757s. Our aircraft and most high performance aircraft have a SAT (static air temp) gage and a TAT (total air temp) gage. The TAT is the sum of the SAT (the true OAT) and the temp rise due to aerodynamic heating. As we approached the final approach fix, we maintained 170 KIAS and 3500' MSL and guess what... The TAT read 4 deg C higher than the SAT!

I can now consider my hypothesis to be correct... At our typical RV cruise speeds, our OAT gages will read about 4 deg C higher than the actual OAT and thus I'm sure my fuselage will stretch a few millimeters as I accelerate to 170 kts! I don't think I 'll be needing any of the space shuttle's thermal tiles or exotic metals, but I do find 4 deg C to be significant if not interesting. The 4 deg C difference will have a minor effect on true airspeed calculations and anything else where temperature is a factor. While saying my RV-8 is like the Concorde and SR-71 is a "strectch" (pun defintely intended), I am astonished that our little airplanes can go fast enough to cause a temp rise! I hope the next time you observe your OAT rise due to increasing airspeed it puts a smile on your face like it does me.

Jerry
N84JE

[Radomir]

Running to the nearest ceramic tile store.. ....

I guess folks with leaky fuel tanks need not worry.. at cruise they'll seal themselves shut anyways

[dhammer]

Being and old B707 engineer I used to spend a bit of time cross checking the gages to stay awake on long oceanic flights. The tool of the trade was the CR-3 round whiz wheel for the calculations. It factors in the ram rise for any speed.

The old gray matter is not like it used to be but a rule of thumb I used was to subtract 50 from the mach number to get the rise.e.g. M.80 = 30 degrees rise. Let's see now - 170kts, thats somewhere about M.23. Nope doesn't work there!! Another was to take the IAS and divide by 10. Sounds a bit closer. Anybody got a CR-3 handy?

Looking around the net I found -

RAM RISE - The rise in air temperature that occurs at high speed. This is due to the compressibility of the air. The faster you go, the more the temperature rises. This can be seen on your OAT gauge. At about 180 knots indicated airspeed, you will notice about a 3? Centigrade rise in the temperature above the actual temperature of the air at that altitude. At about Mach .77 (440 kts TAS, 220 kts IAS at 41,000 Ft) the temperature rise will be about 25? Centigrade. This is why the OAT in a Lear reads -29? or so at cruise instead of -54?, the actual temperature. There are tables where you can calculate the SAT, or static air temperature if you know the "indicated" or Ram Air Temperature and the mach number.

Don

[bsacks05]

Hey Jerry,

I guess your new call sign could be "Luke". (short for lukewarm...get it? )

[lorne green]

Originally Posted by bsacks05

Hey Jerry,

I guess your new call sign could be "Luke". (short for lukewarm...get it? )

Luke Skywalker
SR-71, Millenium Cruiser

[hevansrv7a]

I accept Jerry's information and find it surprising but perhaps it will explain some other stuff. My question to all the EFIS makers - especially GRT because I have one - is: are you compensating for this in your calculations? If not, then is percent power reading high or low? GRT, I know, allows TAS calibrations to be based on GPS, but it's not altitude specific.

[Kevin Horton]

Jerry is correct. At 180 kt TAS, if the OAT probe recovered all the available ram temperature rise, it would indicate a temperature of about 4.3 deg C higher than the ambient temperature. In the real world, the OAT probe's recovery factor will be something less than 1, so it won't read quite that high. If the probe's recovery factor was 0.9, it would see 90% of that ram rise, and would read about 3.8 deg C too high.

There are two ways to calculate ram temperature rise - one way using Mach, and the other using TAS. Both approaches are mathematically equivalent, and they yield the same answer.

If we have TAS in kt, and temperatures in deg C:

IOAT = OAT + ((K * TAS^2)/7592)

OAT = Outside air temperature
IOAT = indicated OAT (i.e. the value you actually see on the OAT indicator)
K = OAT probe recovery factor - usually somewhere between 0.8 and 1, if the probe is truly stuck out in the airflow.

The air entering the engine's induction air system will be heated up by the same effect as it matches speed with the aircraft. So we don't need to worry too much about this effect when using indicated OAT to calculate engine power.

[the_other_dougreeves]

I'm glad you have some good comparison to the SR-71, cuz I'd hate to see your RV traveling at Mach 3+ ...

TODR

[John Clark]

Following Don's lead, I dusted off my Jeppesen CR3 and ran some numbers. The bottom line is that regarding TAS, unless you are in the flight levels, going like stink, temperature rise doesn't make enough difference to read on the "whiz wheel." Even with my good reading glasses on, running numbers from my '8' shows the correction is close to nothing.

John Clark
RV8 N18U "Sunshine"
KSBA

[RV8Squaz]

Yes the corrections would be essentially neglible. In my original post, I wrote the temp difference would have a "minor" effect. I figure TAS would be affected by about 2 kts at my normal cruise speed and altitude and the % horespower would be off by slightly less than 1%. The Lycoming manual states a 1% horespower difference for every 10 deg F. The 4 deg C rise I experienced is slightly less than that.

I have known about compressibility and aerodynamic heating for a long time as it was a flight planning factor in many of the aircraft I flew in the AF. For the T-37 and T-6 which were 270 KTAS aircraft, we didn't apply any corections as they were small and negligible. I was just surprised that I saw a small but noticeable affect on my RV-8 and finally solved the mystery as to why my OAT seemed inaccurate. It didn't dawn on me until recently that it was due to compressibility effects! Of course this was never a factor in other light aircraft I have flown. I'm not going Mach 3+, but I can pretend pretty well as I pass cars on the interstate going 3x faster than they are!

Thanks to Kevin H and the other guys for backing me up with my numbers!

Kevin I look forward to seeing your bird up in the air soon!

Take care,
Jerry